This invention concerns thermoforming forming stations and processes. Thermoforming is a process of forming preheated plastic sheet material by using differential fluid pressures to conform the sheet material to mold surfaces. Twin sheet thermoforming involves the thermoforming of two sheets which are then fused together to form a completed part. This has been done in manufacturing automotive fuel tanks for example, in which the top and bottom tank halves are formed from separate sheets which are then fused together along abutting flanges on each sheet. This process has particular advantages when used for manufacturing fuel tanks as described in U.S. published patent application 2002/0017745 A1.
Various apparatus and processes have been devised for carrying out such twin sheet thermoforming.
Copending application U.S. Ser. No. 10/036,316, filed on Dec. 26, 2001 describes a forming station for twin sheet thermoforming involving simultaneous forming of two sheets and subsequent fusing of the two formed sheets in a single forming station to produce a hollow part. Mold and plug assist tools on one platen are shifted simultaneously with respect to tooling on the other platen in order to align the molds containing two formed sheets in preparation for fusing flanges on the sheets by pressing the same together.
This arrangement is an improvement over an apparatus shown in U.S. Pat. No. 6,372,176 which shows transfer of tooling between separate forming stations since two separate forming presses are required. Another arrangement is shown in U.S. Pat. No. 5,658,523 which uses independent actuators for the plug assist and mold, with the plug assist moved away from the platen when the molds are aligned. In this case, a separate actuator for vertical motion of the plug assist is also required.
U.S. Pat. No. 5,814,185 assigned to the same assignee as the present application describes an improved arrangement for carrying out precision uniform fusing of two thermoformed sheets using hydraulic cylinders and adjustable locking shafts able to be coupled together at various relative positions of the platens.
If the plug assist and mold tools are mounted together for lateral movement on the platen by linear bearings, the high fusing pressure could damage these bearings, which already must bear the heavy weight of the tools, when the hydraulic cylinders are activated to fuse the sheets together.
This results since the hydraulic pressure acts to draw the platens together, which pressure is felt by the bearings supporting the tool set shiftable on the platen. Also, shifting of the tools on the platen requires a much larger platen, increasing its cost and mass which necessitates larger bearings, motors, etc.
The aforementioned copending application also describes shifting of the upper plug assist and mold tools. The upper mold retains one of the formed sheets, and the lateral shifting movement of the mold results in an increased possibility of the formed sheet dropping out of the mold during this shifting movement.
It is the object of the present invention to provide a forming station and process for twin sheet thermoforming which minimizes the number of actuators necessary to carry out the process. It is a further object to provide an improved forming station in which the bearings supporting movable tools are not loaded by the fusing pressure.
It is still another object to make such a twin sheet thermoforming process more trouble free.